This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Shovel dippers are formed with teeth at their leading edge and a dipper door that normally closes the rear of the dipper to hold earth and other materials that are loaded into the dipper by the action of the shovel. The dipper door must be held closed while the dipper is being loaded and while the load in the dipper is swung to a deposit point. At that point, the dipper door is opened to allow the contents of the dipper to empty. The door is typically held closed by a mechanical latch, and is released by a cable or trip wire rope to allow the door to swing open under its own weight and the weight of the contents of the dipper. The door is re-latched by allowing it to swing closed by virtue of its own weight and the changing attitude of the dipper as the dipper rotates back in preparation for its next loading cycle. Dippers typically are equipped with a braking device on the dipper door linkage to reduce the swing speed as the door swings open and closed.
Conventional dipper door braking devices typically utilize a spring loaded bolted connection. As the door swings open or closed by virtue of its own weight, the bolted connection provides friction, reducing the swing speed of the dipper door. The bolted connection is typically tightened to a predetermined torque value to achieve the necessary friction braking force. However, the bolted connection tends to lose its torque value as the braking device wears. To maintain the predetermined torque value, the bolted connection must be adjusted regularly, resulting in machine downtime.
An example of another conventional dipper door braking device can be found in U.S. Pat. No. 6,467,202, issued Oct. 22, 2002, for “Dynamically Active Dipper Door Mechanism.” This patent discloses a door that is controlled by a brake pivotally linking the door to the dipper. The brake may be controlled by linkage actuated by a linear actuator. This conventional dipper door braking device must be actuated to provide a braking force to the dipper door. Therefore, if not actuated, the braking device may permit the dipper door to swing freely by virtue of its own weight, potentially damaging the dipper or other machine components.